1. Field of the Invention
The invention relates to reversible addition-fragmentation chain transfer reagents (RAFT reagents). More particularly, the present invention relates to organometallic RAFT reagents.
2. Description of the Related Art
Recently, with increased maturity of polymer technologies, polymer applications are involved in not only traditional plastics and synthetic resins industries but also high technology industries such as electronics, optoelectronics, communications and biotechnologies. Some relative polymer materials with specific properties are critical for the related industries. For example, photo resistant reagents for preparation of nano-type devices and nano-polymer hybrid materials for dramatically enhancing mechanical properties are all much sought after.
The properties of polymer materials are dependent on configuration. For example, polymerization degrees, molecular weight distribution and components thereof behave in relation to the performance of the polymer materials. The traditional active cation and anion polymerization methods can be used to control the polymerization degree of some monomers and narrow distribution of molecular weight, however, they are limited in their ability to precisely modify the configuration of polymer products. As well, the variety of monomers applied to the above polymerization methods is limited, and strict reaction conditions thereof also restrict use in related industries.
Free radical polymerization is a recently developed technique for controlled polymerization and is widely used in present industry. Nearly 50% of polymer materials such as styrenes, acrylates, methyl methacrylates (MMA) or acrylonitrile (AN) are polymerized in such way. In addition, reaction conditions of free radical polymerization are also milder compared to those of active cation and anion polymerization due to processing under organic solvents. After removal of oxygen gas and other stabilizers, the free radical polymerization can proceed in water.
Since traditional free radical polymerization methods are unable to control polymerization degrees of polymer products, wider molecular weight distribution of the polymer products is observed and the polydispersity index (defined as the ratio of the weight average molecular weight to number average molecular weight, Mw/Mn) is generally more than 2.
Accordingly, it is necessary to develop a novel polymerization method, in which polymerization degree of polymer products is controllable, in order to provide polymer materials with desired configuration and narrow molecular weight distributions and further explore application of polymer materials.
In 1998, CSIRO disclosed an active free radical polymerization method called reversible addition-fragmentation chain transfer process (RAFT process) to prepare polymer products with narrow molecular weight distribution and further control the polymer chain length. The so-called RAFT process is a combination of general procedures for traditional free radical polymerizations with the addition of a fixed amount of reversible addition-fragmentation chain transfer reagent (RAFT reagent). In general, RAFT process controls most free radical polymerizations for alkene monomers.
However, it is very inefficient to polymerize some monomers, such as acrylate, through RAFT process, since such monomers produce polymerization only under very dilute solution conditions to provide polymer products with narrow molecular weight distribution. Since the above problem causes longer reaction time and incomplete reaction of polymerization, it is neither economical nor convenient.
Furthermore, demands and significance of polymer materials having terminal organometallic functional groups are steadily on the increase due to application thereof to dispersants, hybrid materials, optoelectrical materials and nanotechnologies. However, polymer materials having terminal organometallic functional groups are unable to be prepared through conventional RAFT reagents. Therefore, it is necessary to develop a novel RAFT reagent for preparing polymer materials having terminal organometallic functional groups with controllable polymerization degree.